Google was the first place I looked, and after almost an hour of searching I searched all the forums I could too...

I can find lots of places to download rev13 or rev13b but not rev13a.

I downloaded a torrent posted in another topic on this forum but it didn't say which 13 it was and I don't know how to check which one is installed. All I do know is that the Indiana game that I'm trying to get working still doesn't work and the only thing I'm missing is the elusive rev13a.

Maybe I'm missing something here and it's not a single download or something, in which case any constructive comments and help are most welcome

Depends really. 13(a) will get Ghostbusters & Indiana jones working for you. If you really want to play those games, then yes. You'll want 13a. However, 13b has a bug for some people which requires you to re-boot after every game you play via USB. So if you get that bug and that annoys you, you can jump back to 12.